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October 1935 Radio-Craft
 [Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
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Here is a brief synopsis on the
main difference between glass and metal vacuum tubes - the metal case tubes generally
exhibit higher interelectrode capacitances. It showed up in a 1935 issue of
Radio-Craft magazine at the time when metal encased tubes were first appearing.
Unless successfully addressed, that limits usefulness in high frequency circuits.
Of course in 1935 most radio circuits maxed out in the tens of megahertz, so it
was not too much of a problem. One of the major advantageous features of metal tubes
is the built-in EMI/RFI shielding both for keeping desirable fields inside the tubes
and keeping undesirable fields from entering the tube.
Metal vs. Glass Radio Tubes
 Major differences of the new metal and older
glass tubes, including inter-electrode capacities, are discussed.
Albert A. Bombe*
Metal tubes wherever they are shown and discussed, raise the question, "How do
metal tubes compare in characteristics with our present glass types?"
With some basis of fact it might be said that the new metal tubes arrived somewhat
ahead of the new need in radio tube design. Consequently the metal tubes follow
glass types in general characteristics and differ principally in capacities and
the division of certain special-purpose tubes which have two sets of elements in
the glass types and are divided into separate metal tubes each containing one set
of elements.
Several of the new metal tubes almost duplicate in characteristics glass tubes
popular in receiver design during the last year - for example:
6J7 - The metal 6J7 closely matches the glass 77.
6K7 - The metal 6K7 is similar to the glass 78.
6A8 - The metal 6A8 has characteristics matching the 6A7.
6F6 - The metal 6F6 matches the glass 42.
5Z4 - The metal 5Z4 rectifier has a rating similar to that applied to the old
familiar glass 80.
6H6 and 6F5 - The glass 75, popular since its introduction, has been divided
for metal tube production with the diode section appearing in the new metal 6H6,
and the triode elements comprising the new metal 6F5.
6C5 - The general-purpose metal triode, type 6C5, is similar in application to
the glass 76 but in this case the amplification factor of the metal tube was raised
to approximately 20 for improved operation as an oscillator or voltage amplifier.
6D5 - The new metal 6D5 corresponds roughly to the glass 45 in application but,
as in the case of the 6C5, the amplification factor is appreciably increased. In
addition, the metal 6D5 is a unipotential cathode-type tube with a 6.3 V. heater.
The filament-type cathode used in the 45 is not well adapted to the metal tube design.
Direct Inter-Electrode Capacities
6L7 - The most noticeable difference between the comparable characteristics of
the metal tubes and the glass-tubes in modern use appears in the new metal mixer
tube, type 6L7. As explained elsewhere in this issue, this pentagrid tube is not
intended for use as a combination first-detector and oscillator but rather as an
improved mixer working with a separate beat oscillator, such as the 6C5. The advantages
gained with the 6L7 include higher gain and greatly improved isolation of the beat
oscillator and signal-grid circuits. In addition, the 6L7 can be used as an R.F.
amplifier tube with two control grids in operation. This usage results in improved
automatic volume control and much better control-grid cut-off characteristics than
have been available with glass types heretofore.
Metal tubes have produced new engineering problems both in the design, construction
and production of the tubes themselves as well as in the receivers with which they
are to function. Many of the constructional changes necessary in receiver design
are created by the differences in capacity between similar-type metal and glass
tubes. Hence, the comparative capacity data chart - Direct Interelectrode Capacities
- shown on this page will be found invaluable as a reference when experimenting
with these tubes.
*Sales Eng. Dept., Raytheon Production Corp.
Posted July 14, 2023
(updated from original post
on 10/30/2015)
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